1. Field of the Invention
The present invention relates to a process for producing 2,3,3′,4′-biphenyltetracarboxylic dianhydride which is useful as a raw material for polyimide; particularly to a process for producing efficiently the powder product of high purity from a reaction mixture formed by a dimerization reaction of an o-phthalic acid diester.
2. Description of the Related Art
3,4,3′,4′-Biphenyltetracarboxylic dianhydride (hereinafter, may be abbreviated as s-BPDA) and 2,3,3′,4′-biphenyltetracarboxylic dianhydride (hereinafter, may be abbreviated as a-BPDA) are each known as a monomer raw material for production of aromatic polyimide. In particular, a polyimide produced from s-BPDA as a monomer component is superior in properties such as heat resistance, electrical insulation, film strength, film dimensional stability, solvent resistance and the like. Accordingly, because of a high demand for s-BPDA, an efficient process for producing a high purity s-BPDA has been intensively investigated. However, as to a-BPDA, there is no known process for producing a high purity product efficiently. Further, even if polymerized with the same aromatic diamine, a-BPDA and s-BPDA produce polyimides having entirely different properties. Hence, as to a-BPDA, a high-purity product containing no s-BPDA is needed.
For example, JP-B-1994-2715 (patent literature 1), discloses a process which comprises removing unreacted monomers and high-boiling products from a reaction product mixture of a dimerization reaction of an o-phthalic acid diester, and then subjecting the resulting mixture to a crystallization operation in an alcohol solvent such as methanol or the like, thereby high-purity 3,4,3′,4′-biphenyltetracarboxylic acid tetraester (hereinafter, may be abbreviated as s-BPTT) is obtained by the isolation from its isomer, i.e. 2,3,3′,4′-biphenyltetracarboxylic acid tetraester (hereinafter, may be abbreviated as a-BPTT). s-BPTT can be converted into s-BPDA by being heated in the presence of concentrated sulfuric acid in a water-free state, as described in, for example, JP-B-1994-96570 (patent literature 2). In the patent literature 1, however, there is no description on a process for obtaining high-purity a-BPDA from a by-product, a-BPTT.
With respect to the process for obtaining a 2,3,3′,4′-biphenyltetracarboxylic acid tetraester (a-BPTT) as a main product, there is, in JP-A-2003-113143 (patent literature 3), a description that a reaction mixture of high a-BPTT/s-BPTT ratio can be obtained by dimerization of an o-phthalic acid diester in the presence of a particular catalyst. In the literature, there is further described that a-BPDA can be obtained by hydrolyzing a-BPTT and then heating the hydrolyzate at a high temperature for dehydration thereof; however, there is no specific description on the process for obtaining this a-BPDA.